Abstract: Our brain displays an astonishing degree of plasticity. Experiences from a constantly changing environment generate, modify and eliminate synapses and alter the function of our neurons. Extensive research over the last three decades has demonstrated that long term potentiation is a process that requires enduring changes in gene expression. Although transcription factors mediate most of these changes, it is the covalent modifications on DNA and chromatin that render this changes long lasting. Among these, "so called" epigenetic changes, DNA methylation is the only one that cannot be enzymatically reversed. DNA methylation on CpG islands is a well established mechanism of gene silencing. Here, we show that we discovered a novel epigenetic modification, the methylation of CpA dinucleotides. Using a novel, genome-wide method to detect CpA methylation in primary neurons, we made the remarkable observation that CpA methylation appears only on actively transcribed genes. Moreover, our preliminary data suggest that this modification can be modulated by neuronal activity;exactly like the transcription status of the genes that it marks. An irreversible modification that can enhance, or modulate gene expression could have profound consequences in neuronal plasticity. Therefore, we propose experiments that will dissect the role of CpA methylation in gene expression and neuronal function. Public Health Relevance: In this proposal we describe a novel epigenetic modification, the activity dependent methylation of CpA dinucleotides in primary neurons. We propose experimental strategies that will reveal the role of this modification in gene expression and neuronal function. CpA methylation, as a regulatory mechanism, could have critical impact on a plethora of neuronal functions, including axon targeting and synaptic plasticity and specificity. Taken into account that DNA methylation is pharmacologically amenable, our findings could have significant clinical consequences. A large spectrum of neurological or neurodevelopmental disorders, ranging from dementia to autism spectrum disorders, could be caused by the inability of a neuron to transform synaptic activity into long lasting changes in gene expression. Therefore, understanding the role of this epigenetic modification in this process should have a broad scientific, medical and socioeconomic effect.